As disclosed in PTL 1 and PTL 2, in a commonly known hydraulic shock absorber, a liquid chamber in a cylinder housing is filled with oil such as mineral oil, and a piston for braking is housed in the liquid chamber while leaving a flow path gap for the oil around the piston, and a rod connected with the piston extends to the outside from one end of the cylinder housing. When a moving object collides with the distal end of the rod and the piston is displaced, a kinetic energy of the moving object is absorbed by a flow resistance of the oil flowing through the flow path gap.
In the shock absorber having such a known configuration, the rod connected with the piston extends from one end of the cylinder housing so that the moving object which collides with the distal end of the rod is stopped in a shock-absorbing manner. However, when the moving object moves in a reciprocating motion, a pair of shock absorbers oppositely oriented in the movement direction of the moving object needs to be provided in order to stop the reciprocating motion in either direction in a shock-absorbing manner. Further, in a case where the moving object to be stopped in a shock-absorbing manner is, for example, driven to reciprocated by a hydraulic pressure driving device, it is necessary to ensure an installation space for a pair of shock absorbers on or around the hydraulic pressure driving device. Accordingly, the configuration of the hydraulic pressure driving device may be complicated or installation of the pair of shock absorbers may be restricted.
In order to solve the above problem, for example, PTL 3 discloses a buffer (shock absorber) which includes a pair of independent piston rods with one end of each extending from each end of the cylinder in the axis direction of the cylinder.
The buffer disclosed in PTL 3 is practically configured by combining two oppositely oriented buffers having a single piston rod. Since a fluid urged by the piston in both buffers is guided to a separately provided contracting section which has a flow path having an adjustable cross sectional area without being guided to the back side of the piston through a gap around the piston as described in PTL 1, a flow path resistance is independently adjustable in each buffer. Accordingly, a flow path that allows the fluid to flow to the back side of the piston where a negative pressure is generated by movement of the piston in the cylinder is separately formed so that a braking force by each piston rod is independently variable. As a consequence, the flow path for a fluid such as oil that applies a flow resistance to a pair of pistons of the buffer is extremely complicated and is not easy to manufacture, which makes it difficult to stably perform a shock-absorbing function for a long period of time.
Further, in the buffer which is configured to stop the reciprocating motion of the moving object in either direction in a shock-absorbing manner, the moving object collides with the end of one of the piston rods and is stopped in a shock-absorbing manner, and then the moving object collides with the end of the other of the piston rods. Accordingly, when the moving object collides with the end of one of the piston rods, the end of the other of the piston rod needs to be returned to an extended position (return position). However, in a case where a pair of piston rods extending from each end of the cylinder is independent from each other and the other of the piston rods is moved to the return position by a fluid such as oil sealed in the buffer as disclosed in PTL 3, the other of the piston rods may not return to the appropriate return position when the moving object collides with one of the piston rods, due to leakage of a fluid, flow problem of the fluid in a part of the flow path or any other reasons. As a result, an initial shock-absorbing function cannot be performed.
Further, as described above, in a configuration formed by two buffers combined with each other in which braking forces of two piston rods are independent and variable, it is relatively difficult to provide a structure commonly used by the two buffers. In addition to that, since the flow resistances of the fluid urged by the pistons are independently adjustable as described above, the flow path for the fluid such as oil that applies a flow resistance is extremely complicated and a large number of flow paths is necessary. As a result, the overall configuration increases in size and at least size reduction is difficult.